(1) Field of the Invention
The present invention relates to a dry etching method applied in the manufacturing field of semiconductor devices, and particularly to a dry etching method for etching a silicon compound layer whereby high selectivity, high etchrate, low damage and low pollution can be achieved in processing a minute contact hole.
(2) Description of the Related Art
As large-scale integration and high performance of semiconductor devices as seen in recent VLSI have proceeded, higher processing precision is required for dry etching of a silicon compound layer represented by silicon oxide (SiO.sub.2).
First, the enlargement of the area of the device chip due to large-scale integration has increased the size of the wafer. Uniform processing within the wafer surface due to high minuteness of the pattern to be formed is required. Also, multiple-kind small-quantity production as represented by ASIC is required. On the basis of these backgrounds, the main stream of dry etching has been shifting from the traditional batch processing toward the single wafer processing. In this case, for maintaining productivity equal to that achieved in the past, the etchrate per wafer must be improved significantly.
In addition, with a shallow junction of an impurity diffused region for attaining high etchrate and minuteness of the device, and thin material layers of various types, an etching technique whereby higher selectivity to the underlying layer and lower damage than in the past is required. For instance, an SiO.sub.2 interlayer insulation film is etched with a silicon substrate or a polysilicon layer as an underlying layer, when a contact is to be formed in an impurity diffused region formed within a semiconductor substrate or in source-drain regions of a PMOS transistor employed as a resistive load element of SRAM.
Etching of an SiO.sub.2 based material layer has been carried out conventionally in a mode of increased ionicity for cutting a rigid Si--O bond. Typical etching gases are CHF.sub.3, CF.sub.4 and the like, and incident ion energy of CF.sub.x.sup.+ released therefrom is used. However, for carrying out high-rate etching, it is necessary to increase the incident ion energy, and the etching reaction becomes close to a physical sputtering reaction. Therefore, high etchrate and selectivity have always been incompatible with each other.
Conventionally, H.sub.2 or a depositional hydrocarbonaceous gas is added to the etching gas, so as to increase an apparent ratio of the number of carbon atoms to the number of fluorine atoms or C/F ratio. Thus, deposition of carbonaceous polymer generated competitively with the etching reaction is accelerated, thereby achieving high selectivity.
Instead of these conventional etching gases, the present applicant previously proposed a dry etching method for etching a silicon compound layer using a saturated or unsaturated high-order chain fluorocarbonaceous gas having two or more carbons, in the Japanese Patent KOKAI Publication Serial No. 3-276626. The proposed dry etching method is aimed at attaining high etchrate by using a fluorocarbonaceous gas such as C.sub.2 F.sub.6, C.sub.3 F.sub.8, C.sub.4 F.sub.10, C.sub.4 F.sub.8 and the like, thus efficiently forming a large quantity of CF.sub.x.sup.+ from one molecule. However, the single use of the high-order chain fluorocarbonaceous gas increases the amount of formed F*, and the selection ratios to the resist and to the underlying layer cannot be set sufficiently large. For instance, in case of etching an SiO.sub.2 layer on a silicon substrate using C.sub.3 F.sub.8 as an etching gas, though high etchrate is achieved, problems remain such as low selectivity to the resist of about 1.3, shortage of etching durability and generation of dimensional losses due to retreat of the pattern edge. Since selectivity to silicon is about 4.2, a problem remains also in overetching durability.
Thus, in order to solve these problems, two-stage etching is carried out in the above-mentioned prior art, wherein etching with a single use of a high-order chain fluorocarbonaceous gas is suspended shortly before exposure of the underlying layer, and wherein, in etching a remaining portion of a silicon compound layer, a hydrocarbonaceous gas such as ethylene (C.sub.2 H.sub.4) is added to the compound so as to accelerate deposition of carbonaceous polymer. The object of this technique is to supply C atoms in the etching reaction system and to increase the apparent C/F ratio by consuming excessive F* with H* formed in a plasma for forming HF.
However, under the status quo wherein design rules of the semiconductor device is highly minute, dimensional losses from the etching mask is hardly allowable. Even though two-stage etching as mentioned above is carried out, it is necessary to further improve selectivity in the etching of the first stage. Also, as further minuteness proceeds, effects of particle pollution due to carbonaceous polymer may become serious. Therefore, it is preferable to minimize the amount of a depositional gas such as a hydrocarbonaceous gas in the etching of the second stage.
In view of the above-mentioned status of the art, the present inventor proposed a technique wherein a silicon compound layer is etched, with the temperature of a substrate to be etched being controlled to not higher than 50.degree. C., using a chain unsaturated fluorocarbon compound having at least one unsaturated bond within a molecule, in the specification of the Japanese Patent Application No. 2-295225. The chain unsaturated fluorocarbon compound is, for example, octafluorobutene (C.sub.4 F.sub.8), hexafluoropropene (C.sub.3 F.sub.6) and the like. Theoretically, since these gases form two or more CF.sub.x.sup.+ from one molecule on dissociation due to electric discharges, high-rate etching of SiO.sub.2 becomes possible. Also, with the unsaturated bond within the molecule, highly active radicals tend to be generated by dissociation, and polymerization of carbonaceous polymer can be accelerated. Furthermore, since the temperature of the substrate to be etched is controlled to not higher than 50.degree. C., the deposition of the carbonaceous polymer can be accelerated.
With this technique, selectivity to the resist and to the silicon underlying layer was improved significantly without using the depositional gas, and the particle pollution was reduced.
Further, the present inventor previously proposed a technique wherein an etching gas containing a saturated or unsaturated fluorocarbon compound having a cyclic portion at least in part of a molecule structure is used, in the specification of the Japanese Patent Application No. 3-40966. The cyclic fluorocarbon compound has at least three carbons, and has a higher C/F ratio than a chain fluorocarbon compound with the same number of carbons. Therefore, high-rate etching due to a large quantity of CF.sub.x.sup.+ and highly selective etching due to efficient formation of polymer become possible.
In this manner, the above-mentioned chain unsaturated fluorocarbon compound or the cyclic fluorocarbon compound rendered it possible to carry out highly selective etching of a silicon compound layer using an etching gas of single composition.
However, in order to provide a process which can be adapted to production of a future ULSI device, it is necessary to further improve selectivity to the resist for the following reason. That is, in such a large-scale integration device, long-time overetching is indispensable due to an increase of surface steps of the silicon compound layer to be etched. At this time, dimensional losses are generated unless the resist is prevented from retreating.
As another problem, it is necessary to further reduce the particle pollution. The technique using the chain unsaturated fluorocarbon compound, the cyclic fluorocarbon compound and the like has no difference from the conventional technique in that the mechanism for securing selective ratios is attained by deposition of carbonaceous polymer proceeding competitively with the etching reaction. Therefore, if the number of wafer processings is increased, carbonaceous polymer is stored in the etching chamber, thereby deteriorating the particle level. Accordingly, under the status quo wherein the particle pollution is reduced, there is only limited improvements such as a reduction in the frequency of maintenance for cleaning the etching chamber.